Shoulder wear has spread
across to the main ribs.
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Tell us about this small groove.
Its fundamental job is to help prevent the initiation and spread of shoulder edge wear.
But, before we go into that, let’s explain why the Defense Groove design is even needed.
All tires are subject to irregular wear, but it’s much more noticeable on slow wearing tires, like you’d find on tractor-trailers that run ultra-long distance.
Irregular wear is often the result of uneven pressure. The closer you are to the edge of a rib or shoulder, the less pressure there tends to be. The pressure differences may be tiny, but over tens of thousands of miles, these tiny differences can cause shoulder edge wear. It’s not the fault of the tire; it’s simply the nature of things.
What does the edge wear look like?
On a solid shoulder rib tire, it’s a small depression, generally just about the width of your finger, give or take a bit, wearing evenly and at the same rate as the rest of the ribs and grooves.
Most likely, however, the depression widens, marching closer to the main ribs.
The Defense Groove creates a fence, controlling the movement of this depression, so irregular wear cannot spread over to the ribs. It’s easier to understand how a Defense Groove design works if you visualize it as a rib or a fence, rather than an empty groove.
How so?
The Defense Groove design creates a narrow rib along the shoulder of the tire. The pressure is less here, and the rib wears more rapidly than the shoulder rib beside it.
This increases the pressure from the pavement at the very edge of the shoulder rib, helping to equalize pressure across the entire rib – just what we need to promote even wear – to equalize pressure.
As the tire wears, the defense “rib” wears more rapidly than the shoulder rib, continuing to equalize the pressure on the shoulder rib edge.
Why wouldn’t it wear slower?
Consider this: imagine a dual assembly but one tire has a smaller diameter than the other. Because it has a smaller diameter, the smaller tire doesn’t press against the pavement as hard and because it has a smaller circumference (as a result of its smaller diameter), it takes more revolutions to cover the same distance. Since both tires are bolted together, they have to make an identical number of revolutions to cover a given stretch of road.
The only way the smaller tire can do that is to slip and skid as the larger tire drags it along. The big tire wears out the smaller tire, while the smaller tire acts as a brake on the larger tire.
Like the shoulder?
Exactly. The closer you are to the edge of the shoulder, the less pressure there tends to be. The Defense Groove design creates a narrow rib – much like the smaller tire – along the shoulder of the tire. In a sense it sacrifices itself to protect the rib next to it for continued irregular wear protection.
Why isn’t this feature found on all rib tires?
Tires built for metro, pickup-and-delivery and other short hauls are rarely affected by shoulder irregular wear. Let’s face it, turning and maneuvering simply scrubs off most irregular wear that develops.
That’s why a Defense Groove design isn’t found on all rib tires. It depends on the application.
For example, let’s say a long-distance steer tire is mounted by mistake on a truck normally reserved for pick up and delivery. The Defense Groove design may be torn away fairly quickly due to curbing, running over debris and even scrubbing. Again, it’s not the fault of the mis-mounted tire; it’s the brutal work environment and another reason why P&D tires are specially built.
That’s why the Defense Groove feature is an essential irregular-wear-fighting structure for rib radials used in long-distance hauling. Without it, many steer and trailer tires wouldn’t be able to log quite as many miles.
Is the Defense Groove feature found on trailer tires?
Some of Bridgestone’s rib tires are all-position radials, which means they’re suitable for both steer and trailer axle positions. Others are engineered as steer tires while others are built as free-rolling trailer tires.
As long as the tires remain on a long-distance or regional hauling schedule, the Defense Groove structure does a terrific job fighting the initiation and spread of shoulder wear – whether it’s on a steer, all-position or trailer radial. |
